In the manufacture of paper, a stock of fibers and mineral fillers suspended in water, is deposited onto the moving wire on the Fourdrinier table of a paper machine. An example of a conventional Fourdrinier table assembly 10 is shown in FIG. 1. The table 10 includes a head box 12 from which a stock suspension is deposited onto a continuously moving wire 14, a breast roll 16, forming unit 18, and a series of gravity foil boxes 20 and vacuum foil boxes 22, a dandy roll 24, a series of suction boxes 26, and a couch roll 28. As the stock suspension moves along the wire 14 and over the foil boxes 20, 22 and suction boxes 26, the water is removed to form a continuous web.
Many theories have been applied to enhance water removal and achieve proper fiber orientation and distribution to form the fiber sheet, but with varying degrees of success. In one practice, table rolls have been used to apply a vacuum pulse by drawing water from the undersurface of the wire, and then create a pressure pulse by pushing water through the fabric to agitate the stock suspension for proper fiber orientation. However, as production speeds increased and higher vacuum forces were applied, excessive jumping of the stock of the forming sheet occurred which adversely affected formation quality. With the development of hydrofoils, control of water removal and formation improved.
From 1960 to 1970, machines became faster and wider, and the gravity foil box was introduced. The device consisted of a bridge-like framework that spanned the table with “T” bars installed for the individual blades. Foil blades could be removed or added on the run, and the spacing of the “foil banks” was random at best. The concept of foil angle was then proposed and experimentation was performed to determine optimal foil blade angle and foil bank spacing on the machine, which are important to drainage and formation.
A subsequent development was the concept of table harmonics, an engineering principle stating that the energy contained within the stock at the exit of the head box can be amplified (for improved drainage and formation) by the spacing of the foils. The harmonic excitation of the stock can be further altered by placing foil banks at specific intervals along the table based on the tip-to-tip spacing of the foils within each bank. This principle gave rise to the practice of placing the start of a first foil bank in the vicinity of three to six feet from the exit of the head box. It was also learned that the ability to add or remove foils from a bank significantly impacted sheet properties. However, foil banks could not be moved while the machine was running due to the tremendous drag imparted onto the foils. In about 1978, the concept of table frequency was combined with table harmonics to maximize drainage and formation. It was discovered that packing a table with foils spaced an appropriate distance apart, and then removing the foils from the table in strategic locations, achieved the desired Fourdrinier frequency when operating at higher speeds, up to 3300 fpm and higher.
Another development included the introduction of an automated foil bank that varied the pitch of the foil blade (the variable angle foil) to impact drainage and formation. It was also determined that the best formation and drainage for any given table was a frequency between 55 Hz and 105 Hz. In addition, a foil bank system was introduced that could raise foils into the wire and/or drop them from contact with the wire, but only allowed the use of a finite number of frequencies (i.e., either 55 or 75 Hz) by the papermaker. This limits the success of the papermaker where another frequency (i.e., 61 Hz) would be optimal for formation and drainage.
The function of the Fourdrinier table is two-fold: (1) to de-water the stock utilizing the effects of both gravity and applied vacuum, and (2) to subject the stock to periodic excitation as the wire passes over a series of inverted continuous hydrofoil blades (foils) that extend transversely across the table in a cross machine direction, i.e., at a right angle to the direction in which the wire travels.
Traditionally, a Fourdrinier table include several sections of foil groupings, or sets, of approximately six foils each, that are mounted on individual foil support beam structures (i.e, T-bar mounts) spaced along the length of the table at set intervals to create a desired pulse frequency. The foil sets are normally affixed to a sub-structure of the table commonly referred to as a “box.” An example of a conventional foil box 32, having four foils 34 is shown in FIG. 2. The direction of the movement of the wire (not shown) over the foils 34 is shown by arrow 30. The boxes are further sub-classified into either gravity boxes 20 or vacuum boxes 22 (FIG. 1). The first several foil sets aid in de-watering the stock under the influence of gravity. Further down the table as the water content of the stock decreases, a vacuum is applied from beneath the wire to facilitate the de-watering process.
The foils aid in the de-watering process and also impart a pressure impulse to the stock suspension. The impulses serve to keep the fibers and fillers in suspension during the de-watering process yielding a paper stock of uniform consistency. A single pulse is not adequate to control the stock on the Fourdrinier table. Rather, a series of pulses is generated and repeated at a standard interval.
The frequency of these impulses is referred to as the Fourdrinier frequency, which is defined as the velocity of the wire (in inches-per-second) divided by the pitch distance between the foils (in inches). It is well known to those versed in the art/science of papermaking that the frequency of these impulses has a dramatic effect upon the formation of the paper fibers. Under most circumstances, acceptable formation occurs at a Fourdrinier frequency between about 55 hertz and about 90 hertz. However, the current state of the art/science of paper formation relies upon the strategic use of conventional foil blades, multi-pulse foils, and/or foil boards that compromise effective stock de-watering with appropriate stock excitation frequencies.